During the last few decades an intensive effort has been made to discover new, clinically useful antibiotics. Although more than a thousand antibiotics have been discovered, only a few dozen find significant clinical use. To be useful as a chemotherapeutic agent, a substance must have a low toxicity for host cells and a high toxicity for the disease causing microorganism. In other words, the antibiotic must poison the parasite and cause little or no damage to the cells of the host. It is for this reason that a substantial number of the known antibiotics are unsatisfactory as chemotherapeutic agents. In other words, they are not selective in their action on cells and thus interfere with the natural defense mechanisms. Certain disease entities remain serious problems and some of the major antibiotics have considerable drawbacks in terms of limited antimicrobial spectrum or serious side effects. These factors necessitate a continuing search for new antimicrobial agents.